Unidirectional capacitor microphones are provided with a baffle arranged around a microphone unit for lengthening the inter-terminal distance between a front acoustic terminal and a rear acoustic terminal to improve sensitivity (Japanese Utility Model Application Publications Nos. H06-58696 and H06-77394).
According to this method, enlargement of the diameter of the baffle allows the sensitivity to be improved. In contrast, the enlargement causes a diffraction effect in a low frequency band, which sometimes degrades directional frequency response.
Instead, attachment of an acoustic tube, which is typically used for a narrow directional microphone, on a side of the front acoustic terminal of the microphone unit equivalently increases the distance between the acoustic terminals, thereby allowing high sensitivity to be achieved.
However, in the case where an acoustic resistance added to an opening of the tube wall of the acoustic tube is high as with that of a narrow directional microphone, the directivity is narrowed in a high frequency band. Furthermore, this configuration is equivalent to that in which a short tube is connected to the front acoustic terminal. Accordingly, the acoustic capacity according to the internal capacity of the acoustic tube and the acoustic mass of the acoustic tube cause resonance, which resultantly degrades directional frequency response.
It is therefore an object of the present invention to provide a unidirectional microphone that has an excellent directional frequency response and high sensitivity.